This invention relates to a force sensor and a force measurement means which is provided with the force sensor and which is especially suited for use both in stationary and also in transportable wheel load scales.
1. Prior Art
Force sensors (also called dynamometer cells) which are suitable for measuring a force which acts at a point or a point force exist in a host of different embodiments. But for use especially for wheel load scales however force sensors are desired which measure not a force acting at a point, but the integral of a force which acts on a line or on a surface.
CH 667 329 (Haenni and Cie AG) discloses scales with force sensors in the form of elongated hollow elements which are suited for measuring a force which acts in a line. By arranging several of these force sensors in succession the scales can measure the surface integral of the force acting on a surface according to CH 667 329. To measure the force, the volume of a liquid is measured which is displaced by the force on the elastic hollow elements out of them. The scales according to CH 667 329 are well suited as a wheel load scales, but their production is structurally complex and relatively expensive.
2. Description of the Invention
The object of this invention is to make available a force sensor which is especially suitable for measuring the wheel loads of moving vehicles and which is invulnerable to ambient influences such as temperature changes, electromagnetic interference, etc., and which is simple to produce, and a force measurement means or scales provided with the force sensor.
EP-A2-0 141 731 (SFERNICE SOCIETE FRANCAISE DE L""ELECTRO-RESISTANCE) describes a force sensor with an elastically deformable elongated rod with a series of strain measurement elements on its outside along one or more helical lines. The opposite lengthwise ends of the rod are made for clamping in two elements which can move relative to one another in order to measure the bending, torsion and shear stresses acting between these two elements. The force to be measured and the counterforce are supplied at points on the lengthwise ends of the rod. The force sensor as claimed in EP-A2-0 141 731 is not suited for use for wheel load scales where forces acting in a line or on a surface must be measured.
The force sensor as claimed in the invention has an elastically deformable elongated carrier which absorbs he force to be measured and on which there are a first and a second strain measurement element arranged such that the elastic deformation of the carrier which is caused by the force leads to different elongations in the strain measurement elements. This difference of elongations is used a as measure for the force to be measured.
The approach as claimed in the invention has the advantage that disruptive ambient effects (for example, in the form of temperature changes) which influence the two strain measurement elements in the same way do not significantly impair the measurement result since to measure the force only the difference between the measured values of the two strain measurement elements, but not their absolute values, is used.
Preferably the carrier has an elongated shape and is arranged with one side resting essentially over its entire length on a solid, unyielding base such that the force to be measured acts essentially transversely to the lengthwise direction of the carrier and perpendicular to the base on the carrier so that the carrier is compressed in the direction parallel to the action of the force and is stretched in the direction perpendicular to the action of the force and perpendicular to the lengthwise direction of the carrier. The two strain measurement elements can be arranged on the carrier such that the compression is measured essentially by the first strain measurement element and the extension is measured essentially by the second strain measurement element.
The carrier can have a tube (measurement tube) with an essentially circular cross section; in its cavity as overload protection there is a rod element with an essentially circular cross section of a solid material, the diameter of the rod element being dimensioned such that the inner surface of the measurement tube strikes at least partially the surface of the rod element when the force sensor is overloaded before reaching the elastic limit of the measurement tube, by which irreversible, plastic deformation of the measurement tube is prevented.
In one preferred embodiment of the force sensor as claimed in the invention the first strain measurement element in the form of a first quasi-helical line which is coaxial to the measurement tube and the second strain measurement element in the form of a coaxial second quasi-helical line are arranged to run around the axis of the carrier, the shapes of the quasi-helical lines, that is the shapes projected onto a plane normal to the axis, each having one long diameter and one short diameter and the long and the short diameter of the first quasi-helical line being sloped by an angle relative to the long and short diameter of the second quasi-helical line. Preferably the arrangement is such that the angle of incline between the long diameter of the first quasi-helical line and the long diameter of the second quasi-helical line on the one hand and the angle of incline between the short diameter of the first quasi-helical line and the short diameter of the second quasi-helical line on the other each measure essentially 90xc2x0. Advantageously the two quasi-helical lines are arranged in the manner of a two-start thread.
In another embodiment of the force sensor as claimed in the invention the first strain measurement element is located in a first line which meanders along the axis of the measurement tube, the first meander line running essentially on the jacket surface of a first quarter segment of the cylindrical measurement tube. The second strain measurement element is located in a second line which meanders along the axis of the measurement tube, the second meander line running essentially on the jacket surface of a second quarter segment of the cylindrical measurement tube, i.e. the segment adjoining the first quarter segment. The strain measurement elements arranged in a meander shape thus run in surfaces along the tube axis which are essentially perpendicular to one another. The meander lines can run in a zig-zag, sawtooth, sinusoidally, semicircularly, rectangularly or in some other meandering shape. Furthermore, there can be a third strain measurement element in a third meander line essentially on the jacket surface of a third quarter segment which borders the second quarter segment, and a fourth strain measurement element in a fourth meander line essentially on the jacket surface of a fourth quarter segment between the first and the third quarter segment. In this arrangement with four strain measurement elements the first can be serially connected to the third and the second can be serially connected to the fourth.
In one embodiment with a carrier which lies with one side essentially over its entire length on a solid, unyielding base, an alternative embodiment of the invention, the carrier has a top which is provided with first ribs, a bottom which is provided with second ribs and which is essentially parallel to the top, a left side and a right side. The second ribs are arranged essentially parallel to the first ribs and are offset to them. The first strain measurement element is located on the right side of the carrier and runs in a zig-zag between the top and the bottom back and forth, its running from the first rib on the top to the next rib on the bottom, from the latter to the next rib on the top, from it to the next rib on the bottom, etc. The second strain measurement element is located on the left side of the carrier and runs in a zig-zag between the top and the bottom back and forth, its running from the first middle between the two ribs on the top to the next middle between the two ribs on the bottom, from it to the next middle between the two ribs on the top, etc. For a force acting essentially perpendicular to the top on the first ribs the carrier which lies essentially with the second ribs on an unyielding base is deformed in an undulatory manner, the first strain measurement element being compressed and the second strain measurement element being stretched. In contrast to the carrier which lies with one side essentially over its entire length on a base the carrier which lies simply with ribs on the base is deformed not simply it is transverse direction, but also in its lengthwise direction.
In another preferred embodiment of the force sensor the strain measurement elements are arranged in grooves which are made in the surface of the carrier. The strain measurement elements can be sealed with an electrically insulating material in the grooves. If the strain measurement elements consist of an ordinary resistance wire, the sealing of the latter in a groove has the advantage that not only tensile stress but also compressive stress can be measured by means of the resistance wire. The sealing material causes compressive forces to be transferred to the resistance wire. In contrast, by means of an exposed resistance wire compressive stress cannot be measured since the wire deflects as a result of the compressive forces. If nevertheless not only elongations but also compressions of the groove are to be measured with a resistance wire which is located exposed in a groove, the resistance wire must be prestressed such that it is always exposed to a tensile stress when the groove is being compressed.
Preferably the strain measurement elements have an electrical conductor which has electrical resistance which is dependent on the elongation. The electrical conductor can be for example an ordinary electrical resistance wire, constantan being preferred as the material for the resistance wire. To measure the electrical resistance ratio from the first to the second electrical conductor as a measure of the force to be measured, the force sensor as claimed in the invention preferably has a Wheatstone bridge circuit. This embodiment of the force sensor as claimed in the invention has an extremely short reaction time; this is especially advantageous when wheel loads of moving vehicles are being measured.
In another preferred embodiment of the invention the carrier and the strain measurement elements are located in a hermetically sealable overtube which consists preferably of an electrically conductive material. The force sensor as in this embodiment of the invention is protected especially well against external effects such as temperature changes, vibrations, electromagnetic interference, moisture, dirt, etc.
The force measurement means as claimed in the invention has one or more force sensors as claimed in the invention. Preferably the force measurement means has at least two force sensors with one first and one second strain measurement element each, which each have one electrical conductor with an electrical resistance which is dependent on the elongation, and a single Wheatstone bridge circuit, for determination of the total force which acts on the force sensors by means of the Wheatstone bridge circuit the electrical conductors of the first strain measurement elements being located in a first series connection and the electrical conductors of the second strain measurement elements being located in a second series connection.
In one preferred embodiment of the invention, wheel load scales have the force measurement means as claimed in the invention, one or more of the elastically deformable carrier being located essentially in a straight line and lengthwise to the direction of travel of the wheels to be weighed. The wheel load scales as in this embodiment of the invention can have a coupling device for coupling to other connectable wheel load scales. In this way several of these wheel load scales can be coupled to one another in order to cover the entire width of the roadway without adulteration of the measurement result and/or dead areas forming at the coupling sites.
The description of this invention which is detailed below in conjunction with the attached drawings is used as only one example for better understanding of the invention and should not be interpreted as a limitation of the protected scope of the claims. For one skilled in the art other advantageous embodiments and combinations of features which however still lie within the scope of this invention can be easily seen from the following description in conjunction with the attached drawings and the totality of patent claims.